One cutting accessory used to perform a surgical procedure is the bur. A bur generally consists of a head formed from rigid material, typically metal, shaped to have a number of flutes. The flutes are formed to define cutting edges. More particularly, the flutes are typically designed to cut hard tissue, such as bone or cartilage. A shaft extends rearwardly from the head. The proximal end of the shaft often has a feature that facilitates locking the shaft to a powered handpiece. The actuation of the handpiece results in the rotation of the bur. During a surgical procedure, the bur head is placed against a surgical site where a section of tissue is to be removed. The rotating cutting edges excise tissue away from the surgical site. Burs of various shapes and sizes are used in procedures such as orthopedic surgery, neuro and spinal surgery, ear noise and throat surgery and in other surgical procedures in which a sub-procedure is to selectively remove a section of tissue.
Burs work well for the purposes for which they are designed. Nevertheless, a problem associated with some burs is chatter. Chatter is the back and forth vibration of a bur head against the surface to which the bur head is applied. Chatter occurs as a result of bur's individual cutting edges repeatedly being forced against the tissue against which the bur head is applied. There are a number of reasons a bur may chatter.
One reason a bur chatters is because it receives an input of energy due to a process known as regeneration of waviness. This process is due to the fact that, when a cutting edge passes across a section of tissue, it leaves a specific wavy (essentially sinusoidal) profile along the surface of the tissue. If two adjacent cutting edges cut in phase, the second cutting edge excises tissue along a surface profile identical to that along which in was excised by the first flute. In practice, due to the invariable movements of the bur head and the tissue, this does not happen. When any two successive cutting edges pass over the same tissue section, the second flute cutting edge removes tissue on a path that does not overlap the tissue wave formed by the chip excised by the first cutting edge. Consequently, the debris chips cut by the second cutting edge have variable thicknesses. This means, during the process in which the second cutting edge excises the chip from the tissue, the cutting edge and its flute are subjected to variable forces. Over time, the repetitive exposure of the bur flutes to these variable forces causes the bur to undergo forced vibration. This vibration results in bur chatter.
The Applicant's Patent Application U.S. Pat. Pub. No. 2008/0132929 A1, the contents of which are incorporated herein by reference, discloses a bur with flutes designed to reduce chatter. The bur of this invention includes multiple flutes. More particularly, this bur is shaped so that some, but not all, of the flutes are formed so that their cutting edges emerge from the body the bur head very close to the distal end tip of the bur head. The remaining flutes are shaped so that their cutting edges emerge from the bur head at positions more proximal to the distal end tip of the bur head. This bur has been found to be somewhat effective in reducing bur chatter.
Nevertheless, even a bur constructed with the above features can, in some circumstances, chatter.